Manager Davey Johnson said today that infielder Mark DeRosa, who has been sidelined since April 29 with a strained left oblique, has been cleared to go on a rehab assignment, likely at Single-A Potomac.

The discomfort in the oblique is still lingering, but DeRosa’s going to try to play through it.

“I’m going to let him go out, and for me, as much as experience as he’s got, if he can go through a couple days without aggravating it any worse, I’ll activate him,” Johnson said.

Reliever Brad Lidge went a perfect inning at Potomac last night, striking out two. He came out of that outing “feeling great,” Johnson said.

The Nats’ skipper also said reliever Ryan Mattheus made it through today’s pitchers’ fielding practice session “with flying colors” and has been cleared to throw a simulated game at Potomac tomorrow.

He’ll then make a rehab appearance on Sunday and could be ready to rejoin the Nats early next week, if all goes well.

“He actually has said that he feels better pitching on his foot now than he did at any time during the previous two months,” Johnson said. “He’s way ahead of schedule.”

The Nats will face knuckleballer R.A. Dickey today, which might not thrill some of the guys in Johnson’s order. We know Danny Espinosa isn’t happy about it; he has said in the past he hates facing Dickey.

So what’s the key to facing a knuckleball pitcher? Most of us have heard the old saying, “If it’s high, let it fly, if it’s low, let it go,” but Johnson gave some more in-depth information on the art of hitting a knuckleball today.

In his playing days, Johnson said he crowded the plate against a knuckleballer and treated it like a breaking ball, waiting for it to drop down and break away from him.

“The key to hitting a knuckleballer is being patient enough to gauge the one you know is probably going to move a certain way in the strike zone,” Johnson said. “They get more hittable the more he misses, but this guy today is also a guy that in the past has used his fastball and slider to get ahead, if he’s not having good command with his knuckleball. They key to hitting and being successful is being patient.”

Dickey has had a fantastic start to the season, going 8-1 with a 2.69 ERA.

As for Espinosa perhaps hoping he might not have to face a knuckleballer, Johnson laughed when informed about his second baseman’s past comments about Dickey.